A theory of nondilute spray evaporation based upon multiple drop interactions

Abstract

A theory of nondilute spray evaporation has been developed which takes into account the separation distance between drops. This theory is based upon the global conservation equations for the two-phase mixture and the conservation equations for a drop evaporating in finite surroundings. Results obtained for n-decane using this theory show that as the equivalence ratio of the two-phase mixture decreases from 200 to 10^(−2), four universal regimes are identified: (1) a regime of lean overall mixtures and dilute sprays where nondilute and dilute spray theories agree, (2) a regime where although both theories predict complete evaporation before saturation, the ratio of the evaporation times (onndilute/dilute) predicted by the two theories is a function of the equivalence ratio, (3) a regime where the nondilute spray theory predicts saturation before complete evaporation, whereas the dilute spray theory predicts the opposite, and (4) a regime where both theories predict saturation before complete evaporation, but at different residual drop sizes. In regimes 1 and 2 the evaporation time is a decreasing function of the equivalence ratio, whereas in regimes 3 and 4 it becomes an increasing function of the equivalence ratio. Parametric variations of the initial gas composition show that departures from the predictions of the dilute spray theory are obtained even for very dilute sprays if the overall mixture is rich. This implies that the dilute spray theory cannot adequately describe evaporation of drops injected during the later part of injection, or of larger-than-average drops in polydisperse sprays. Other departures from the dilute spray theory are obtained for sprays injected into relatively hot gases and for dense sprays initially composed of relatively cold drops. It was also shown that the evaporation time of a spray is a decreasing, nearly linear function of the initial drop temperature.

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